The present invention relates to a process and apparatus for adsorbing moisture from gases, particularly air, and, more particularly, to a method of controlling a duration of an adsorption cycle in the process for adsorbing moisture from gases.
German Published Patent Application 36 25 013 discloses utilization of dried air for drying bulk material, particularly plastic granulates. The dried air is fed through the bulk material dropping through a drying container under gravity in a reverse flow. The moisture carried with the exhaust air from the drying container is removed from the exhaust air by a moisture-adsorbing material. After a fixed time period the moisture-adsorbing material with this moisture is separated and is regenerated by a hot regeneration medium, mostly hot air, during a regeneration cycle.
Usually the duration of adsorption is determined according to the maximum load of moisture which the adsorbing material, mostly molecular sieve, may hold. The greatest possible quantity of moisture from the gas to be dried should be adsorbed during the adsorption period, and the dried air again supplied to the drying container should have a low dew point.
When the drying container is run with maximum throughput through the bulk material to be dried, the exhaust air drying apparatus explained above operates with high efficiency. However, in case the moisture load of the exhaust air decreases, e.g. because the throughput through the drying container is reduced or because the bulk material contains less moisture, the adsorbing material within a drying pot is regenerated after a fixedly set adsorption period although the maximum moisture adsorption capacity of the adsorbing material is not reached.
To improve the energy balance in such cases the dew point of the dried air flowing out from the drying pot and being supplied to the drying container is monitored continuously for extending the adsorption period of the drying pot until the dew point approaches a tolerable limit value. Only when the limit value of the dew point is reached will the drying pot be switched to regeneration.
However, some expenses are involved in providing a dew point sensor of sufficient load tolerances. Moreover the dew point sensors are often subject to failures. Such metering sensors degrade easily and become contaminated already after a short lifetime and emit then inexact signals which prevent full utilization of the moisture adsorbing capacity of the adsorption material. Moreover, the exhaust air may assume high temperatures of approximately 60.degree. through 80.degree. C. when the throughput through the drying container is reduced. At these high temperatures, a molecular sieve no longer can adsorb moisture effectively. Therefore, in case of reduced moisture load of the exhaust air a bad dew point is signalled, although the moisture adsorbing capacity of the molecular sieve is not fully utilized.
For determining the optimum length of the adsorption period U.S. Pat. No. 2,561,441 recommends scanning the temperatures of the air to be dried at the input into the adsorbing material and at the output from the adsorbing the adsorbing material during the adsorption period. A temperature difference is formed from the input temperature and the output temperature which indicates the actually adsorbed moisture at any moment.
When the temperature difference decreases to a preset minimum temperature difference, the drying pot including the adsorbing material is switched to the regeneration phase. This method for switching from the adsorption period to the regeneration period operates as desired only if the moisture content of the input air remains constant in time. However, for a number of granulate drying processes the moisture content of the input air varies. Consequently, when the moisture content of the input air decreases, also the temperature difference decreases. Thereby, an erroneous indication of the saturation of the adsorbing material is obtained to the effect, that the drying pot is switched to the regeneration period although the adsorbing material is not completely utilized.